The effect of stacking of end groups on the rheological behavior of supramolecular polymer melts is reported. Oscillatory shear experiments in the transition zone from the pseudo rubber plateau to the flow region of telechelic polycaprolactones (PCLs) with ureidopyrimidinone (UPy) end groups directly attached to PCL can be fitted with a single Maxwell element. This demonstrates that dimerization of the UPy groups is unidirectional and that reversible chain scission is faster than reptation. If the UPy groups are connected to the polymer via a urethane linker, a low-frequency plateau in G′ is observed. This is ascribed to the formation of a network of stacked UPy dimers, aided by urethane hydrogen bonding. Below their melting point, these stacks form long fibers in the urethane linked supramolecular poly(methyl caprolactone), which were observed with atomic force microscopy (AFM). Steric hindrance interferes with stacking, since the plateau in G′ is lower in a urethane linked polymer with bulky adamantyl-UPy end groups.

The rheology of supramolecular polycaprolactone polymers with two ureidopyrimidinone (UPy) end groups and unimer molecular weights between 600 and 4000 was compared with that of branched tri- and tetrafunctional analogs. Above the melting point of polycaprolactone, the previously observed low-frequency plateau in storage and loss moduli during oscillatory frequency sweep of lower molecular weight bifunctional unimers was shown to persist up to 130−170 °C, where it gradually disappeared but reappeared upon slow cooling. Even though they are supramolecularly cross-linked, the tri- and tetrafunctional materials showed no plateau. This counterintuitive behavior was further investigated with optical microscopy, WAXS, and DSC experiments, which indicated that the plateau is closely connected to the presence of crystalline domains in the lower molecular weight fraction of bifunctionalized unimers. Because the formation of crystallites is prevented by branching, and because the network formed by the tri- and tetrafunctional unimers has a short lifetime, the branched materials do not show a low-frequency plateau.